What is the main function of the inner region of the kidney?

The inner region of the kidney, primarily known as the medulla, plays a crucial role in the concentration of urine. This process is essential for maintaining the body's water balance and ensuring that waste products are efficiently excreted while minimizing water loss.

Within the medulla, the presence of loops of Henle and collecting ducts is vital for creating a concentration gradient. As urine passes through these structures, water is reabsorbed back into the bloodstream due to the higher osmolarity of the surrounding interstitial fluid, a process enhanced by the counter-current multiplication mechanism. This allows the kidneys to produce urine that is more concentrated than the blood plasma, conserving water and helping to regulate hydration levels in the body.

The other functions listed in the choices, such as blood filtration and nutrient reabsorption, primarily occur in other regions of the kidney, particularly the cortex, making the role of the medulla distinctly focused on urine concentration.